Optical pickup device minimizing an undesirable astigmatism

ABSTRACT

An optical pickup device comprises an objective lens unit having an objective lens, a support member for supporting the objective lens and an objective-lens driving mechanism for driving the objective lens in a radial direction and a focusing direction of an optical disc in such a way that a light beam is focused on an information recording surface of the optical disc; and a main unit having an optical irradiation system which includes a light source for emitting a light beam and guides the light beam to the objective lens, and an optical detection system which includes a photo-sensor and guides reflected light from the information recording surface to the photo-sensor via the objective lens. The optical pickup apparatus further has a tilt-position-adjusting mechanism for supporting the objective lens unit on the main unit and tilting an optical axis of the objective lens with respect to an optical axis of the optical irradiation system in such a way as to minimize an undesirable astigmatism given by the optical disc and optical elements in the optical irradiation system and the optical detection system.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to an optical pickupdevice for use in an optical information recording/reproducing apparatusthat records and/or reproduces signals from an optical informationmedium such as an optical disc.

[0003] 2. Description of the Related Art

[0004] For a recording/reproducing apparatus for recording/reproducinginformation on an optical disc, loaded therein, such as an optical videodisc, a digital audio disc, and so on, a focus servo and a trackingservo are essential for always accurately converging light beams forwriting and reading information to a pit train or the like formedspirally or concentrically on a recording surface of the optical disc.The focus servo performs a positional control for an objective lens,used to irradiate a pit train on the optical disc with light beams, inan optical axis direction so as to reduce a focus error, i.e., an errorof the position of the objective lens in the optical axis direction withrespect to the focus position of the objective lens. The tracking servoperforms a positional control for the position of the objective lens,used to irradiate a pit train on the optical disc with a light beams,with respect to a recording track in a radial direction of the opticaldisc, so as to reduce a tracking error, i.e., an error of the objectivelens with respect to the pit train recording track position.

[0005]FIG. 1 illustrates a conventional optical pickup device using theastigmatism method.

[0006] A laser beam from a semiconductor laser 1 is transformed into aparallel laser beam by a collimator lens 2, passes through a polarizingbeam splitter 3, and is converged by an objective lens 4 toward anoptical disc 5 to form a light spot onto a pit train on an informationrecording surface of the optical disc 5.

[0007] Light reflected from the optical disc 5 is converged by theobjective lens 4 and directed by a beam splitter 3 to a detecting lens7. A converged light beam formed by the detecting lens 7 passes througha cylindrical lens 8, serving as an astigmatism generating element, toform a spot image near the center ‘O’ of a light receiving surface of aquadrant photodetector 9 having four light receiving surface areas(elements) divided by two orthogonal line segments. The cylindrical lens8 irradiates the quadrant photodetector 9 with a light spot SP in theshape of true circle as illustrated in FIG. 2A when the laser beam isconverged on the recording surface of the optical disc 5 in focus, andan elliptic light spot SP, extending in an orthogonal direction of theelements as illustrated in FIG. 2B or 2C when the converged laser beamis out of focus on the recording surface of the optical disc 5 (FIG. 2Billustrates the light spot SP when the objective lens 4 is too far fromthe optical disc 5, while FIG. 2C illustrates the light spot SP when theobjective lens 4 is too near the optical disc 5), thus generatingso-called astigmatism.

[0008] The quadrant photodetector 9 opto-electrically transduces thelight spot irradiated to the four light receiving surface areas intorespective electric signals which are supplied to a focus errordetecting circuit 12. The focus error detecting circuit 12 generates afocus error signal (FES) based on the electric signals supplied from thequadrant photodetector 9 and supplies the focus error signal to anactuator driver circuit 13. The actuator driver circuit 13 supplies afocusing driving signal to an actuator 15. The actuator 15 moves theobjective lens 4 in response to the focusing driving signal in theoptical axis direction.

[0009] The focus error detecting circuit 12, as illustrated in FIG. 3,is connected to the quadrant photodetector 9, where the quadrantphotodetector 9 is composed of four detecting elements DET1 to DET4 infirst to fourth quadrants which are located adjacent to each other withtwo orthogonal division lines L1 and L2 interposed therebetween andwhich are independent of each other. The quadrant photodetector 9 ispositioned such that the division line L2 is in parallel with atangential direction with respect to the extending direction of therecording track, and the other division line L1 is in parallel with theradial direction of the same. Respective opto-electrically transducedoutputs from the elements DET1 and DET3, symmetric with respect to thecenter ‘O’ of the light receiving surface of the quadrant photodetector9, are added by an adder 22, while respective opto-electricallytransduced outputs from the elements DET2 and DET4, also symmetric withrespect to the center ‘O’ of the light receiving surface, are added byan adder 21, and outputs from the respective adders 21 and 22 aresupplied to a differential amplifier 23. The differential amplifier 23calculates the difference between the supplied signals, and outputs asignal indicative of the difference therebetween as a focus error signal(FES).

[0010] As described above, in the conventional focus error detectingcircuit 12, the outputs of the quadrant photodetector 9 are added by theadders 21 and 22, respectively, and the differential amplifier 23calculates the difference between the outputs of the adders 21 and 22 togenerate a focus error component. In this event, when the light beam isin focus, the light spot in the shape of true circle as illustrated inFIG. 2A is formed on the quadrant photodetector 9, where a spotintensity distribution is symmetric with respect to the center ‘O’ ofthe light receiving surface of the quadrant photodetector 9, i.e.,symmetric in the tangential direction and in the radial direction, sothat the values resulting from the additions of the opto-electricallytransduced outputs from the elements on the diagonals are equal to eachother, with the focus error component being calculated to be “zero”. Onthe other hand, when the light beam is out of focus, i.e., an ellipticlight spot extending in a diagonal direction as illustrated in FIG. 2Bor 2C is formed on the quadrant photodetector 9, so that the valuesresulting from the additions of the opto-electrically transduced outputsfrom the elements on the diagonals are different from each other. Thus,the focus error component output from the differential amplifier 23exhibits a value corresponding to the focus error. Specifically,assuming that the references designated to the elements of the quadrantphotodetector 9 represent the outputs thereof, the focus error signalFES is expressed by the following equation:

FES=(DET1+DET3)−(DET2+DET4)

[0011] Since the objective lenses of conventional CD players have asmall numerical aperture and a large focal depth, slight noise ifappeared on a focus error signal (FES) would be negligible as a focuserror. In a case where information is read from an optical disc havinglands and grooves, such as a DVD-RAM, however, the numerical aperture ofthe objective lens becomes larger and the focal depth thereof becomesshallower, so that the influence of noise contained in the focus errorsignal on the focus servo of the objective lens becomes greater.

[0012] In conventional optical pickup apparatuses which read informationfrom an optical disc having pre-grooves, therefore, the focus servosystem cannot follow up noise on an FES, thus raising problems likeoscillation of the focus servo circuit and heating of the actuator.

OBJECT AND SUMMARY OF THE INVENTION

[0013] Accordingly, the present invention has been made in view of theproblem mentioned above, and thus an object thereof is to provide anoptical pickup apparatus capable of adequately eliminating noise from afocus error signal that is generated at the time a light spot crosses atrack or groove in the astigmatism method, particularly, the noisecomponent that is originated from a further astigmatism of the opticalsystem or birefringence of the substrate of an optical disc.

[0014] An optical pickup device according to the present inventioncomprises an objective lens unit having an objective lens, support meansfor supporting the objective lens and an objective-lens drivingmechanism for driving the objective lens in a radial direction and afocusing direction of an optical disc in such a way that a light beam isfocused on an information recording surface of the optical disc; a mainunit having an optical irradiation system which includes a light sourcefor emitting a light beam and guides the light beam to the objectivelens, and anoptical detection system which includes photosensing meansand guides reflected light from the information recording surface to thephotosensing means via the objective lens; and tilt-position-adjustingmeans for supporting the objective lens unit on the main unit andtilting an optical axis of the objective lens with respect to an opticalaxis of the optical irradiation system in such a way as to minimize anundesirable astigmatism given by the optical disc and optical elementsin the optical irradiation system and the optical detection system.

[0015] In the optical pickup device having the structure abovementioned, said tilt-position-adjusting means includes a supportingstructure for fixing the objective lens unit at least two points ontosaid main unit after the adjustment of position for tilting saidobjective lens unit is performed on the main unit using at least onescrew at one of the points.

[0016] In the optical pickup device having the structure abovementioned, said supporting structure is that said objective lens unit isfixed on said main unit at three points which do not lie in a straightline, at one point of which said objective lens unit is anchoringly incontact with the main unit, and at remaining two points of which theadjustments of position for said objective lens in the tilting areperformed with two screws, so that the optical axis of the objectivelens is tilted toward one of a pair of regions including the verticalangles of the first to fourth quadrants defined by the tangential andradial directions of the optical disc all around the optical axis of theoptical irradiation system on the main unit.

[0017] In the optical pickup device having the structure abovementioned, said supporting structure is that said objective lens unit isfixed on said main unit at two points, at one point of which saidobjective lens unit is anchoringly in contact with the main unit, and ata remaining points of which the adjustments of position for saidobjective lens in the tilting are performed with a screw, so that theoptical axis of the objective lens is tilted toward one of a pair ofregions including the vertical angles of the first to fourth quadrantsdefined by the tangential and radial directions of the optical disc allaround the optical axis of the optical irradiation system on the mainunit.

[0018] Another optical pickup device according to the present inventioncomprises:

[0019] an objective lens;

[0020] support means for supporting said optical lens;

[0021] an objective-lens driving mechanism for driving said objectivelens in a radial direction and a focusing direction of an optical discin such a way that a light beam is focused on an information recordingsurface of said optical disc;

[0022] an optical irradiation system which includes at least one lightsource for emitting a light beam and guides said light beam to saidobjective lens;

[0023] an optical detection system which includes photosensing means andguides reflected light from said information recording surface to saidphotosensing means via said objective lens; and

[0024] shift-position-adjusting means for shifting an optical axis of alight beam irradiated from said light source with respect to an opticalaxis of said optical irradiation system so that an optical axis of saidobjective lens is tilted with respect to an optical axis of said opticalirradiation system so as to minimize an undesirable astigmatism causedfrom said optical disc and optical elements in said optical irradiationsystem and said optical detection system.

[0025] In the optical pickup device having the structure abovementioned, said shift-position-adjusting means including screwingmechanisms for moving the optical axis of the light source in directionscorresponding to the tangential and radial directions of the opticaldisc from the optical axis of the optical irradiation systemindividually.

[0026] In the optical pickup device having the structure abovementioned, said shift-position-adjusting means including a screwingmechanism for moving the optical axis of the light source in a directionwhich does not correspond to the tangential and radial directions of theoptical disc from the optical axis of the optical irradiation system.

[0027] In the optical pickup device having the structure abovementioned, said optical irradiation system including; a collimator lensconverting a diverging light beam irradiated from the semiconductorlaser to a parallel light; and a complex prism guiding the parallellight beam to the objective lens while shaping a cross-section of thelight beam and having a partial function of the optical detection systemas to guide a reflected light from the optical disc to said opticaldetection system.

[0028] In the optical pickup device having the structure abovementioned, said photosensing means which comprises; an astigmatismgenerating element giving astigmatism the reflected light; and aquarter-split photosensor having at least four elements arranged in apoint symmetrical to one another with respect to the center of alight-receiving surface.

[0029] In the optical pickup device having the structure abovementioned, the optical pickup device further comprises adiffraction-grating element which forms two light spots from plus andminus first-order light beams that are irradiated in a point symmetricalfashion around a 0-th order light spot in a middle distance between thetwo light spots.

[0030] In the optical pickup device having the structure abovementioned, the optical pickup device further comprises rotationaladjusting means which rotates the diffraction-grating element withrespect to the optical axis of the optical irradiation system in such amanner that the radial component of the distance between the centers ofthe two light spots becomes ½ of a track pitch of the optical disc.

[0031] In the optical pickup device having the structure abovementioned, said astigmatism generating element is one of selected form acylindrical lens, hologram element and parallel transparent plate.

[0032] The optical pickup apparatus of the present invention adjusts theangular position of the objective lens itself to allow a light beam toobliquely enter the objective lens, providing an image height whichproduces an astigmatism to nearly minimize the undesirable astigmatismof optical elements in the optical irradiation system and opticaldetection system and an optical disc, so that a noise componentoriginated from the astigmatism or the birefringence of the substrate ofthe optical disc can be adequately eliminated from the focus errorsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a schematic diagram illustrating the structure of anoptical pickup device;

[0034]FIGS. 2A, 2B and 2C are plan views each illustrating a variationin shape of a beam spot on a light receiving surface of a photodetector;

[0035]FIG. 3 is a schematic block diagram illustrating a focus errorsignal generator in a conventional optical pickup device;

[0036]FIGS. 4A and 4B are schematic partial perspective views of thesurface of an optical disc and a light beam, illustrating how a lightspot which moves in the radial direction of the optical disc is formed;

[0037]FIGS. 5A through 5D are plan views each showing the distributionof the intensities of light spots on the light-receiving surface of aphotosensor in an optical pickup apparatus which is formed by thereflected light that has caused an oblique astigmatism;

[0038]FIG. 6 is a graph showing the characteristics of noise in an FESfrom the photosensor in the optical pickup apparatus which is generatedby the reflected light that has caused an oblique astigmatism;

[0039]FIG. 7 is a schematic structural diagram depicting a relationshipbetween the angle of view and image height caused by the objective lensof the optical pickup apparatus embodying the present invention;

[0040]FIG. 8 is a schematic partial perspective view showing arelationship between the objective lens of the optical pickup apparatusembodying the present invention and an optical disc;

[0041]FIG. 9 is a schematic perspective view of the optical pickupapparatus of the present invention;

[0042]FIG. 10 is a schematic partial perspective view illustrating arelationship between an optical irradiation system and an opticaldetection system of the optical pickup apparatus of the presentinvention;

[0043]FIGS. 11 through 13 are schematic side views each showing theoptical pickup apparatus of the present invention;

[0044]FIG. 14 is a schematic partial plan view of the optical pickupapparatus of the present invention;

[0045]FIG. 15 is a schematic structural diagram illustrating a principleof an optical pickup apparatus in another embodiment according to thepresent invention for showing a relationship between the angle of viewand the image height based on the light source, the collimator lens andthe objective lens;

[0046]FIG. 16 is a schematic partial perspective view illustrating thelight source and the collimator lens in the optical irradiation systemincluded in the optical pickup apparatus of another embodiment accordingto the present invention;

[0047]FIG. 17 is a schematic partial perspective view illustrating thelight source and the collimator lens in the optical irradiation systemincluded in the optical pickup apparatus of a further other embodimentaccording to the present invention; and

[0048]FIG. 18 is a schematic partial plan view of the surface of anoptical disc having pre-pits recorded on lands.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Preferred embodiments according to the present invention will nowbe described with reference to the accompanying drawings.

[0050] An undesirable astigmatism in an optical pickup apparatus occursin a case of low alignment precision where, for example, the light-beamtransmitting surface of an optical part like a diffraction grating or ahalf mirror is inclined and not perpendicular to the optical axis of anoutgoing light beam or in a case where the outgoing light beam from asemiconductor laser itself has an astigmatism, or an astigmatism is alsoproduced by the birefringence of the disc's substrate which isoriginated from the irradiation and reflection of a light beam.

[0051] An astigmatism component for which a meridional image point orsagittal image point is stretched in the tangential (track) direction orradial direction of an optical disc can be canceled out by, for example,slightly diffusing or converging the light beam that comes into ashaping prism. However, a so-called oblique astigmatism component forwhich a meridional image point or sagittal image point is stretched in adirection of, for example, 45° to the tangential (track) direction orradial direction of an optical disc remains in the overall opticalsystem. When a condensed light beam is irradiated on a disc substratemade of polycarbonate (PC), for example, an astigmatism in an obliquedirection of 45° to the tangential (track) direction or radial directionof the optical disc appears.

[0052] We have checked the noise component which is originated from anastigmatism in an oblique direction of 45° to the tangential (track)direction or radial direction and is produced when a light beam crossesa land and a groove in a case where a focus error signal is acquiredfrom a quarter-split photosensor in an optical pickup apparatus whichreproduces signals from an optical disc having grooves and lands formedon the information recording surface using the astigmatism method.

[0053] First, as shown in FIG. 4A, a light beam is irradiated by anoptical irradiation system on lands 31 and grooves 32 formed spirally orconcentrically on the information recording surface of an optical disc5, forming a light spot SP, the light spot SP is moved in the radialdirection from (a) to (d) as indicated by the broken-line arrow, and acheck is made on FES noise on the focus error signal as the light spotmoves across the track. It is to be noted however that the opticalirradiation system which produces an astigmatism AS in the light spot SPin a direction of 45° to the track direction or the tangential directionas shown in FIG. 4B and a DVD-RAM optical disc comprised of apolycarbonate (PC) disc substrate are used. The groove width and theland width of the optical disc 5 are equal to each other. Thephotosensing means is a quarter-split photosensor 9 which comprises atleast four elements arranged in a point symmetrical to one another withrespect to the center of the light-receiving surface which are placed inthe first to fourth quadrants into which the light-receiving surface isdivided in association with the tangential and radial directions of theoptical disc around the optical axis of the optical detection system asthe center, as similar to FIG. 3.

[0054]FIGS. 5A through 5D show the distribution of the intensities oflight spots to be formed on the light-receiving surface of thequarter-split photosensor 9 in the respective cases where the light spotSP of a perfect circle when focussed is located at positions (a) to (d)in FIG. 4A. In the vicinity of the center of the groove 32, thedistribution becomes as shown in FIG. 5A and dark portions appear in aDET 4 and DET 2 so that the focus error signal (FES) becomes maximumfrom the equation of the FES, FES=(DET1+DET3)-(DET2+DET4). When thelight spot SP further moves and comes near a taper 33 at the boundarybetween the groove 32 and the land 31, the distribution becomes as shownin FIG. 5B and the FES becomes zero. As the light spot SP further movesand comes near the center of the land 31, the distribution becomes asshown in FIG. 5C and dark portions appear in a DET 1 and DET 3 so thatthe FES becomes minimum. As the light spot SP further moves and comesnear another taper at the land-groove boundary, the distribution becomesas shown in FIG. 5D and the FES becomes zero. As the light spot SPfurther moves and comes near the center of another groove, thedistribution becomes as shown in FIG. 5A again, yielding the maximumFES. As apparent from the above, a track cross signal which has themaximum level and the minimum level respectively at the states shown inFIGS. 5A and 5C is produced and this variation becomes FES noise. When alight spot in a focused state crosses a groove and a land, the FESshould be equal to 0 ideally. However, the FES does not become zerobecause of the astigmatism in a direction of 45 to the track(tangential) direction. Therefore, a track cross signal appears on theFES which repeatedly reaches the maximum and minimum levels during thecrossing of spot on the grooves and lands.

[0055] As shown in FIG. 6, for example, as the tracking error signal FTS(DET1+DET4)−(DET2+DET3) and the reproduced signalRFS=(DET1+DET4+DET2+DET3) are detected, the phase of the FES output isdeviated from the phases of those signal outputs, so that such a FESinvites a defocusing of the device.

[0056] As mentioned above, we have reveal that such a deviation of theFES output is caused by the astigmatism in a direction of 45° to thetrack (tangential) direction. It is necessary to reduce the astigmatismin a direction of 45° to the track (tangential) direction in order tocancel out the deviation of the peak of the FES from the peaks of theFTS and RFS for eliminating the defocusing state.

[0057] According to the following first embodiment, the defocusing iseliminated by adequately setting the image height caused by theobjective lens.

[0058] When a parallel light beam enters the objective lens 4 having afocal distance f at an incident angle θ (angle of view) to the opticalaxis, the light beam is condensed on a point at an image height Y asshown in FIG. 7. In this case, the relationship between the angle ofview θ and the image height Y causedby the objective lens 4 is Y=f tanθ. The relationship is generally understood as follows: if an infinitedistant object standing on the optical axis subtends the angle θ at thelens 4, then the image height Y is expressed as the size of the imageformed in the focal plane determined by the chief ray of the light beam.In general, the astigmatism of the lens greatly depends on the imageheight Y caused by the objective lens. That is, the greater the imageheight becomes, the greater the astigmatism becomes. Therefore, theastigmatism in a direction of 45° to the track (tangential) directioncan be canceled out by tilting the optical axis of the objective lens bythe angle of view θ with respect to the focusing direction (i.e., theoptical axis of the optical irradiation system) because of changing theimage height Y caused by the objective lens.

[0059] As shown in FIG. 8, the optical axis of the optical irradiationsystem AIS (which extends in the focusing direction) is setperpendicular to the recording surface of the optical disc 5 (i.e., theplane where the tangential direction and radial direction exist), andthen the optical axis of the objective lens AOL is set to the angularposition of the angle θ with respect to the optical axis of the opticalirradiation system AIS within a plane 45 (two-dotted line) that extendsin a direction of 45° (=φ) from the tangential and radial directions ofthe optical disc 5 including the optical axis of the optical irradiationsystem so as to alter the image height caused by the objective lens 4 ina direction of 45° from the track (tangential) direction. By adjustingthe angle of view θ formed by the AOL and the AIS in the plane 45, theastigmatism in a direction of 45° to the track (tangential) directioncan be canceled out, because the angle of view θ changes the imageheight Y caused by the objective lens. For example, the tilting positionat the angle θ between the AOL and the AIS can be finely adjustedthrough the following steps of; setting the objective lens 4 in aninitial state at which the principal plane of the objective lens isexpected to be parallel to the recording surface of the optical disc 5i.e., making the AOL coincide with the AIS; tilting the objective lens 4from the initial state to a θ R angular position so that the AOLinclines from the AIS at an angle OR within a plane including the AISand extending in a radial direction of the optical disc 5, and furthertilting the objective lens 4 from the θ R angular position to a θ Tangular position within the plane including the AIS and extending in thetangential direction of the optical disc 5 so that the AOL inclines fromthe AIS at the angle θ.

[0060]FIG. 9 shows an optical pickup apparatus embodying such a settingprocess for adjusting the angle of view θ. As shown in the Figure, theoptical pickup apparatus comprises an objective lens unit 40 and a mainunit 50. The main unit 50, that will be described later in details, hasan optical irradiation system which includes a light source like asemiconductor laser for emitting a light beam and guides the light beamto the objective lens, and an optical detection system which includesphotosensing means and guides reflected light from the informationrecording surface to the photosensing means via the objective lens.

[0061] Provided between the objective lens unit 40 and the main unit 50is tilt-position-adjusting means which tilts the optical axis of theobjective lens with respect to the optical axis of the opticalirradiation system. The tilt-position-adjusting means may be structuredas follows: The objective lens unit 40 is provided with two tongueportions projecting in the tangential and radial directions andincluding through-hole portions 41 and 42 respectively, which arepositioned away from the optical axis of the optical irradiation system.The objective lens unit 40 also has a tongue portion including apoint-of-support portion 43 located on the opposite to the optical axisof the optical irradiation system. There are female screw portionsprovided on the top of the main unit 50 correspondingly to the holes ofthe through-hole portion 41 and 42 in such a manner that the opticalaxis of the optical irradiation system approximately coincides with theoptical axis of the objective lens 4. The point-of-support portion 43 isclamped and fixed to the main unit 50 by a flat spring 51 with aL-shaped section screwed on the main unit to be anchored thereon. Inaddition, the point-of-support portion 43 may be directly screwed on themain unit. The objective lens unit 40 is supported at three points onthe main unit 50 by securely holding the point-of-support portion 43 andfastening the objective lens unit 40 at the through-hole portions 41 and42 at tilted positions of a predetermined angle θ by screws.

[0062] The objective lens unit 40 includes the objective lens 4, elasticsupport members 15 like a flat spring, which supports the objective lens4 on the objective lens unit 40, and an objective-lens driving mechanismlike an actuator, which drives the objective lens 4 in the radialdirection and the focusing direction of the optical disc 5 in such a wayas to focus a light beam on the information recording surface of theoptical disc 5. The objective lens 4 supported by the support member 15is mounted on a holder, and the objective-lens driving mechanism has acoil which extends in the radial direction and the focusing directionand a magnetic circuit. The coil and magnetic circuit function incooperation with the holder.

[0063] As shown in FIG. 9, the main unit 50 is secured to a slider 70which moves on a shaft 60 extending in the radial direction of theoptical disc 5. The slider 70 is provided with a tilting mechanism fortilting the main unit 50 at an angle θ C with respect to the axis of theshaft 60 in the plane in the radial direction of the optical disc 5 thatincludes the optical axis of the optical irradiation system. The tiltingmechanism may comprise a fixing section 71, a tiltably fixing section 72and a tilt adjusting section 73. In the fixing section 71, a femalescrew portion is provided on the side portion of the main unit 50 nearthe seat portion for the point-of-support portion 43 and a screw entersthrough a through hole of the slider 70 to the female screw portion. Thetiltably fixing section 72 is located apart from the fixing section 71in the radial direction of the optical disc 5. In the section 72, anelongated female screw portion of the main unit 50 is provided on theside portion of the main unit 50. A screw enters through another throughhole of the slider 70 to the elongated female screw portion. The slider70 is fastened by these screws to the main unit 50. At the tiltadjusting section 73, the position of the main unit 50 at the angle θ Cin the extending direction of the slider 70 or the radial direction ofthe optical disc is set around the fixing section 71 by making thedistal end of a bolt, fastened in a through hole formed in the overhangportion of the slider 70 which is provided apart from the fixing section71 with the tiltably focusing direction 72 in between, in contact withthe corresponding top portion of the main unit 50 and turning the boltto move it in the focusing direction.

[0064]FIG. 10 shows the inside of the main unit 50, as shown the Figure,in which an optical irradiation system and an optical detection systemare provided an a base portion 50 a of the main unit. The opticalirradiation system comprises a semiconductor laser 1 for emittinglinearly polarized light, a collimator lens 2 converting a diverginglight beam irradiated from the semiconductor laser to a parallel light,a polarized-beam splitter 3, a ¼-wavelength plate 18 and a risingreflector 20. The optical detection system comprises the risingreflector 20, the ¼-wavelength plate 18, the polarized-beam splitter 3,a condenser lens 7 for detection, an astigmatism generating element 8like a cylindrical lens, hologram element, parallel transparent plate ormulti-lens, and the quarter-split photosensor 9. The polarized-beamsplitter 3 is a complex prism which guides the parallel light beampassed the collimator lens 2 to the objective lens 4 while shaping across-section of the light beam and having a partial function of theoptical detection system as to guide a reflected light from the opticaldisc to the quarter-split photosensor 9. A diffraction-grating element 6like a 3-beam generating grating may be provided between the collimatorlens 2 and the light-incident surface of the polarized-beam splitter 3which is inclined to the optical path. This diffraction-grating element6 forms two light spots SP from plus and minus first-order light beamsthat are irradiated in a point symmetrical fashion around a 0-th orderlight spot SP₀ in the middle of the distance between the light spots SP,on the information recording surface of the optical disc, as shown inFIG. 18. The optical disc 5 is designed in such a way that recordingpits can be formed thereon in the form of either grooves or lands, andpre-pits have previously been recorded in the form of lands. It ispossible to provide rotational adjusting means which irradiates twolight spots in such a manner that the radial component of the distancebetween the centers of two light spots SP becomes ½ of the track pitchby turning the diffraction-grating element 6 with respect to the opticalaxis.

[0065] As shown in FIG. 10, the linearly polarized light beam emittedfrom the semiconductor laser 1 is transformed into parallel light by thecollimator lens 2, is then shaped by the polarized-beam splitter 3, andis then circularly polarized by the ¼-wavelength plate 18. Thecircularly polarized light is deflected by the rising reflector 20toward the objective lens 4 along the optical axis of the opticalirradiation system approximately perpendicular to the optical disc 5,and is condensed by the objective lens 4, thus forming a light spot onthe optical disc 5. The reflected light from the light spot travelsthrough the objective lens 4 and the rising reflector 20 and reaches the¼-wavelength plate 18 to be linearly polarized. This linearly polarizedlight is reflected by a dielectric multilayer film of the polarized-beamsplitter 3, and is split and guided to the condenser lens 7. This lightthen passes through the astigmatism generating element 8 and enters thequarter-split photosensor 9. The optical head having a good reproductioncharacteristic can be constructed if the direction of the linearlypolarized light (which is parallel to the junction surface in a case ofa semiconductor laser) is perpendicular to the track direction. This isbecause the outgoing angle of the light beam which is perpendicular tothe junction surface is greater than the outgoing angle which isparallel to the junction surface.

[0066] As the optical axis of the objective lens 4 is inclined withrespect to the tangential direction and/or the radial direction of theoptical disc, the astigmatism of the overall optical system from thelight source to the optical disc can almost be minimized by making theastigmatism originated from the image height caused by the objectivelens cancel out the astigmatisms the optical irradiation system, theoptical detection system and the optical disc have.

[0067] The operation of this embodiment will now be discussed.Adjustment of the optical pickup including adjustment of the tiltedposition of the objective lens is performed as follows. After theobjective lens unit 40 and the main unit 50 are assembled as shown inFIG. 11 in the manufacturing procedures, it should be checked that thetracking error signal FTS and the reproduced signal RFS are acquiredsufficiently. Next, it should be checked that the peak of the focuserror signal FES is not deviated from those of the FTS and RFS. If thereis no deviation in the signal peaks, then it is judged that theastigmatism produced in the optical path from the semiconductor laser 1to immediately before the objective lens 4 is relatively small, or thatthe light beam incident to the objective lens 4 is not tilted much tothe optical axis of the objective lens 4 and is condensed at asufficiently low image height as compared with the effective field ofview at the objective lens. If the astigmatism that is generated undersuch conditions is equal to or smaller than 0.01 λ rms, for example,approximately the smallest astigmatism is obtained, then the angularadjustment will not be carried out to complete the adjustment.

[0068] If the peak of the FES deviates from those of the FTS and RFS, onthe other hand, then the tilt position of the objective lens 4 isadjusted to fix the objective lens 4 at the predetermined angle θ bysetting the angular position of the objective lens unit 40 with respectto the main unit 50 to the angle θ T and the angle θ R in the tangentialand radial directions by turning the screws in the tangential and radialthrough-hole portions 41 and 42 in such a way as to minimize thedeviation of the FES, as shown in FIG. 12. Even if the astigmatismproduced by the optical system is large as in this case, the astigmatismof the optical system can be minimized by adjusting and setting theangle θ to the optical axis of the objective lens 4 which sets the imageheight caused by the objective lens in such a manner that theastigmatism originated from the image height caused by the objectivelens 4 cancels out the unwanted astigmatism caused by the other factorsin the optical system. The optical axis of the objective lens 4 to beset to the angular position of the angle θ to the optical axis of theoptical irradiation system can be set not only in the plane that extendsin the direction of 45° from the tangential and radial directions of theoptical disc 5 including the optical axis of the optical irradiationsystem but also in the plane that extends in a direction of such anarbitrary angle as to minimize the astigmatism by adjusting the angle θT and the angle θ R. This embodiment can therefore minimize the obliqueastigmatism of an arbitrary angle other than the oblique astigmatism of45° which is produced in the optical system.

[0069] Thereafter, the main unit 50 is tilted at the angle θ C withrespect to the axis of the shaft 60 by turning the screw at the tiltadjusting section 73 as shown in FIG. 13 to correct the comaticaberration of the objective lens 4.

[0070]FIG. 14 shows a schematically plan view of the optical pickupapparatus of this embodiment in which the surface of the main unit 50 isdivided into four regions of first to fourth quadrants by the tangentialand radial directions of the optical disc around the optical axis of theoptical irradiation system. The objective lens unit 40 is pivotallypreliminarily supported at a point at the point-of-supporting portion 43in the second quadrant region on the main unit and is finally screwedonto the main unit 50 at the two points i.e., the tangentialthrough-hole portion 41 and the radial through-hole portion 42. Theoptical axis AOL of the objective lens 4 is tilted into the second andfourth quadrants including the vertical opposite angles.

[0071] Alternatively, in another embodiment, the objective lens unit 40may be supported at only two points, i.e., the point-of-support portion43 and a support portion 44 (dotted line) in the fourth quadrant region,as shown in FIG. 14 so that the optical axis AOL of the objective lens 4is tilted into the second and fourth quadrants. Further, a support pointfor fixing the objective lens unit may be added. In this way, thistilt-position-adjusting means should have such a structure that theobjective lens unit is supported on the main unit at least two pointsand is screwed into a tilted position at least one point in the oneregion of the first to fourth quadrants defined by the tangential andradial directions of the optical disc around the optical axis of theoptical irradiation system.

[0072]FIG. 15 shows a principle of a second embodiment in which thelight irradiating portion of the semiconductor laser 1 i.e., an opticalaxis of a light beam irradiated therefrom is shifted parallel to theoptical axis of the collimator lens 2 (i.e., the optical axis of theoptical irradiation system) with a distance Δ, by which the light beamincident onto the objective lens 4 is changed by the angle of view θ sothat the objective lens 4 makes an image height generate acounterbalance astigmatism for countervailing an unwanted astigmatismcaused by the optical elements other than per se. In this case, therelation ship between the angle of view θ and the deviation Δ of thesemiconductor laser 1 is Δ=f″·tan θ wherein f″ denotes the focaldistance of the collimator lens 2. Therefore, when the optical axis oflight beam irradiated from the light source is shifted with respect tothe optical axis of the optical irradiation system by a pertinent valueand the shifted position is set, then an wanted astigmatism is minimizedgiven by the optical elements belonging to the optical disc and theoptical irradiation system and the optical detection system. The opticalelements is omitted other than the semiconductor laser 1, the collimatorlens 2 and the objective lens 4 in the optical irradiation system shownin FIG. 15.

[0073]FIG. 16 shows a shift-position-adjusting mechanism in which theoptical axis of the light source 1 for irradiating a light beam isshifted with respect to the optical axis of the optical irradiationsystem by the deviation θ as a shifted position in such a way that thelight source 1 supported in a sliding holder is moved in directionscorresponding to the tangential and radial directions of the opticaldisc 5 from the optical axis of the optical irradiation system by screws11 a, 11 b as screwing mechanisms individually. The rotation of screws11 a moves the light source 1 in the tangential direction of the opticaldisc. The rotation of screws 11 b moves the light source 1 in the radialdirection of the optical disc. Therefore, the optical axis of light beamirradiated from the light source is shifted in parallel from the opticalaxis of the optical irradiation system at an angle φ=45° direction withrespect to the tangential and radial directions of the optical disc andfixed the shifted position with the deviation Δ. The angle φ can be setoptionally.

[0074]FIG. 17 shows another shift-position-adjusting mechanism in whichincludes a sliding holder having slope and supporting the light source 1to be moved in a direction φ=45° direction apart from the tangential andradial directions by rotation of a screw 12 as a screwing mechanism. Inthis way, the shift-position-adjusting mechanisms comprise the screwingmechanism for moving the optical axis of the light source in a directionwhich does not correspond to the tangential and radial directions of theoptical disc 5 from the optical axis of the optical irradiation system.

[0075] According to all embodiments above mentioned, it will beobviously understood that an infinite conjugate type objective lenssystem (a parallel light beam type) may be employed in which aconverging lens is arranged on the side of the optical recording mediumand a collimator lens is arranged on the side of the semiconductorlaser. Instead of the infinite conjugate type objective lens system, afinite conjugate type objective lens system may be employed in which theobjective lens directly converges a diverging light beam irradiated formthe laser onto the optical lens, so that there will be obtained asimilar effect of the optical pickup device in the above embodiments inwhich, by using the tilt-position-adjusting mechanism orshift-position-adjusting mechanism above mentioned, an undesirableastigmatism caused from the optical systems is minimized. In addition,the optical pickup device according to the invention can well reproducesignals from the recorded prepits on the lands of the optical disc.

[0076] According to the present invention, there is provided thetilt-position-adjusting mechanism or shift-position-adjusting mechanismin the optical pickup device, a good performance of the optical pickupdevice with a diffractive limit optic system will be demonstrated sincethe unwanted astigmatism is cancelled as a whole the optical irradiationsystem.

What is claimed is:
 1. An optical pickup device comprising: an objectivelens unit having an objective lens, support means for supporting saidoptical lens and an objective-lens driving mechanism for driving saidobjective lens in a radial direction and a focusing direction of anoptical disc in such a way that a light beam is focused on aninformation recording surface of said optical disc; a main unit havingan optical irradiation system which includes at least one light sourcefor emitting a light beam and guides said light beam to said objectivelens, and an optical detection system which includes photosensing meansand guides reflected light from said information recording surface tosaid photosensing means via said objective lens; andtilt-position-adjusting means for supporting said objective lens unit onsaid main unit and adjusting a position of said objective lens unit andprovided between said main unit and said objective lens unit so that anoptical axis of said objective lens is tilted with respect to an opticalaxis of said optical irradiation system so as to minimize an undesirableastigmatism caused from said optical disc and optical elements in saidoptical irradiation system and said optical detection system.
 2. Anoptical pickup device according to claim 1, wherein saidtilt-position-adjusting means includes a supporting structure for fixingthe objective lens unit at least two points onto said main unit afterthe adjustment of position for tilting said objective lens unit isperformed on the main unit using at least one screw at one of thepoints.
 3. An optical pickup device according to claim 2, wherein saidsupporting structure is that said objective lens unit is fixed on saidmain unit at three points which do not lie in a straight line, at onepoint of which said objective lens unit is anchoringly in contact withthe main unit, and at remaining two points of which the adjustments ofposition for said objective lens in the tilting are performed with twoscrews, so that the optical axis of the objective lens is tilted towardone of a pair of regions including the vertical angles of the first tofourth quadrants defined by the tangential and radial directions of theoptical disc all around the optical axis of the optical irradiationsystem on the main unit.
 4. An optical pickup device according to claim2, wherein said supporting structure is that said objective lens unit isfixed on said main unit at two points, at one point of which saidobjective lens unit is anchoringly in contact with the main unit, and ata remaining points of which the adjustments of position for saidobjective lens in the tilting are performed with a screw, so that theoptical axis of the objective lens is tilted toward one of a pair ofregions including the vertical angles of the first to fourth quadrantsdefined by the tangential and radial directions of the optical disc allaround the optical axis of the optical irradiation system on the mainunit.
 5. An optical pickup device according to claim 1, wherein saidoptical irradiation system including; a collimator lens converting adiverging light beam irradiated from the semiconductor laser to aparallel light; and a complex prism guiding the parallel light beam tothe objective lens while shaping a cross-section of the light beam andhaving a partial function of the optical detection system as to guide areflected light from the optical disc to said optical detection system.6. An optical pickup device according to claim 1, wherein saidphotosensing means which comprises; an astigmatism generating elementgiving astigmatism the reflected light; and a quarter-split photosensorhaving at least four elements arranged in a point symmetrical to oneanother with respect to the center of a light-receiving surface.
 7. Anoptical pickup device according to claim 1 further comprising adiffraction-grating element which forms two light spots from plus andminus first-order light beams that are irradiated in a point symmetricalfashion around a 0-th order light spot in a middle distance between thetwo light spots.
 8. An optical pickup device according to claim 7further comprising rotational adjusting means which rotates thediffraction-grating element with respect to the optical axis of theoptical irradiation system in such a manner that the radial component ofthe distance between the centers of the two light spots becomes ½ of atrack pitch of the optical disc.
 9. An optical pickup device accordingto claim 6, wherein said astigmatism generating element is one ofselected form a cylindrical lens, hologram element and paralleltransparent plate.
 10. An optical pickup device comprising: an objectivelens; support means for supporting said optical lens; an objective-lensdriving mechanism for driving said objective lens in a radial directionand a focusing direction of an optical disc in such a way that a lightbeam is focused on an information recording surface of said opticaldisc; an optical irradiation system which includes at least one lightsource for emitting a light beam and guides said light beam to saidobjective lens; an optical detection system which includes photosensingmeans and guides reflected light from said information recording surfaceto said photosensing means via said objective lens; andshift-position-adjusting means for shifting an optical axis of a lightbeam irradiated from said light source with respect to an optical axisof said optical irradiation system so that an optical axis of saidobjective lens is tilted with respect to an optical axis of said opticalirradiation system so as to minimize an undesirable astigmatism causedfrom said optical disc and optical elements in said optical irradiationsystem and said optical detection system.
 11. An optical pickup deviceaccording to claim 10, wherein said shift-position-adjusting meansincluding screwing mechanisms for moving the optical axis of the lightsource in directions corresponding to the tangential and radialdirections of the optical disc from the optical axis of the opticalirradiation system individually.
 12. An optical pickup device accordingto claim 10, wherein said shift-position-adjusting means including ascrewing mechanism for moving the optical axis of the light source in adirection which does not correspond to the tangential and radialdirections of the optical disc from the optical axis of the opticalirradiation system.
 13. An optical pickup device according to claim 10,wherein said optical irradiation system including; a collimator lensconverting a diverging light beam irradiated from the semiconductorlaser to a parallel light; and a complex prism guiding the parallellight beam to the objective lens while shaping a cross-section of thelight beam and having a partial function of the optical detection systemas to guide a reflected light from the optical disc to said opticaldetection system.
 14. An optical pickup device according to claim 10,wherein said photosensing means which comprises; an astigmatismgenerating element giving astigmatism the reflected light; and aquarter-split photosensor having at least four elements arranged in apoint symmetrical to one another with respect to the center of alight-receiving surface.
 15. An optical pickup device according to claim10 further comprising a diffraction-grating element which forms twolight spots from plus and minus first-order light beams that areirradiated in a point symmetrical fashion around a 0-th order light spotin a middle distance between the two light spots.
 16. An optical pickupdevice according to claim 15 further comprising rotational adjustingmeans which rotates the diffraction-grating element with respect to theoptical axis of the optical irradiation system in such a manner that theradial component of the distance between the centers of the two lightspots becomes ½ of a track pitch of the optical disc.
 17. An opticalpickup device according to claim 14, wherein said astigmatism generatingelement is one of selected form a cylindrical lens, hologram element andparallel transparent plate.